Method and apparatus for thermal gas purging

ABSTRACT

A plasma etching chamber is cleaned with a supply of pressurized nitrogen or argon that is electrically heated. The cleaning cycle is performed during the idle time following an etching cycle. Electrically heated cleaning gas flows into the etching chamber for a predetermined period during which the etching chamber rises. Flow of heated cleaning gas is then stopped, and the etching chamber is evacuated by existing equipment. The cycle can be started automatically using a pressure signal from a conventional purging gas line or from a signal at a SECS/GEM port.

TECHNICAL FIELD

[0001] This invention relates in general to semiconductor processing equipment. In particular, the invention relates to a method and apparatus for cleaning semiconductor plasma etching equipment during the idle time between etching cycles using a gas at elevated temperature and pressure.

BACKGROUND ART

[0002] A common method for etching elements on semiconductor wafers comprises contacting a semiconductor surface having a developed photoresist with an energized plasma that reacts with undeveloped photoresist and the underlying substrate. The photoresist is removed using additional chemicals or plasma and the wafers are inspected to ensure the image transfer from the mask to the top layer is correct. Moisture and material from each etching cycle deposit on the etch chamber, and build up over time. Eventually, this creates a contamination problem and reduces yield. The etching chamber must therefore be cleaned periodically. Most of existing cleaning methods have several drawbacks, the most important one being that the production process must be halted while the chamber is cleaned, which reduces throughput.

[0003] U.S. Pat. No. 5,468,686 issued to Kawamoto on Nov. 21, 1995 discloses a method of cleaning an etching chamber containing an etched wafer. The apparatus requires a separate chamber for etchant removal, and the method employs a combination of gases under strong vacuum while irradiating the wafer surface with radio waves to reduce the etchant to ash. Improved throughput is claimed based on reduced cleaning time over previous methods, but the cleaning step is still performed after the etching step. Also, the apparatus is significantly more complex and expensive to construct and operate than conventional equipment.

[0004] U.S. Pat. No. 5,678,759, issued to Charles A. Grenci et al. discloses an apparatus (hereafter the HGS system) for cleaning a vacuum etching chamber. Improvements on the apparatus and methods for using the apparatus are disclosed in U.S. Pat. No. 5,906,055, U.S. Pat. No. 5,979,075 and U.S. Pat. No. 6,049,997, all of which are also issued to Grenci et al. Both the apparatus and method have been used to clean a LAM TCP 9600 Metal Etcher. In the HGS system, the chamber is purged with a “hot gas sweep” of nitrogen at significantly higher pressures than conventional evacuation type cleaning methods. The nitrogen is heated by compression in a dual, multi-lobe rotor, roots-type blower, and is constantly circulated through the blower, even when sweep gas is not being sent to the etching chamber. The nitrogen's temperature and pressure in combination with the relatively high sweep gas flow rate are claimed to improve cleaning over conventional purge/evacuation methods. However, neither the temperature or the pressure of the gas is regulated. Temperature therefore varies over a substantial range, which reduces cleaning performance, and peak pressures reach a level that would exceed process limit settings. For this and other reasons, the HGS system requires that several of the existing interlocks in the etching chamber process control apparatus be overridden during cleaning. Even when this is handled by a programmable logic controller, several minutes pass between the startup of the HGS system and the time when actual cleaning of the etching chamber can begin. Further, the blower is cumbersome, and the overall apparatus uses up a fairly large amount of the valuable space in a clean environment and is relatively expensive to build and takes several days to install (i.e. also expensive to install). Since interlocks need to be overridden, the HGS system also requires that the etching process be shut down during cleaning. The HGS system so far has been publicly demonstrated as working only with cleaning etching chambers used for metallization layer etching. Data is not generally available on whether the HGS system will work as well with chambers used to etch polysilicon or oxide layers.

[0005] A cleaning method and apparatus that can be performed during the inspection of the etched wafers is greatly desired, since cleaning the etching chamber would not delay the manufacturing process. An apparatus that can be remotely activated by existing equipment and does not require that the existing interlocks be overridden would be a significant improvement. A method and apparatus that controls temperature and pressure of the cleaning gas for optimal performance is also desirable. The apparatus should take up minimal space in the clean room and of course be inexpensive to install and operate.

DISCLOSURE OF INVENTION

[0006] In general, the desired advantages and features are achieved by a method and apparatus that improve on the HGS system by employing an electrically heated once-through gas stream to flush the etching chamber. The gas can be nitrogen or argon, and gas temperature is regulated at a value between about 85° C. and 200° C. depending on the application. The apparatus includes means for detecting when the etching chamber enters an idle state, means for electrically heating a cleaning gas stream, means for controlling the flow of gas to the etching chamber, and a controller to control gas temperature, cleaning gas flow, and evacuation of the etching chamber. Optional means for regulating the cleaning gas pressure are preferred, although critical flow restrictions such as an orifice plate or venturi nozzle can be used to regulate flow rate into the etching chamber, which will indirectly control pressure in the etching chamber. When the apparatus is installed on an existing etch chamber, a pressure sensor can be attached to the existing purge gas line (which is dead-ended at the sensor) to determine when the chamber enters an idle condition. Preferably, the sensor is a pressure switch, although a transmitter can also be used as the pressure sensor. Alternatively, when a SECS/GEM port is available, the idle state signal can be obtained as a digital signal from the SECS/GEM port. The SECS/GEM port usually also provides a separate signal to indicate when the etching process restarts, which can be used to shut off the flow of cleaning gas to the etching gas.

[0007] To perform the method of the invention, the controller waits until it receives a signal that the etching chamber is in an idle state. At that point, a valve is opened to begin flow into the etching chamber. After about thirty seconds, the flow is shut off, and the etching chamber is evacuated using an existing vacuum pump connected to the etching chamber, also for about thirty seconds. This flow/evacuation cycle can be repeated as many times as desired or until the etching chamber is once again put in use. For long idle times, flow/evacuation cycles are performed for about twenty minutes, followed by a rest period of about thirty minutes. The total cleaning cycle time can therefore range from as short as about one minute up to about six hours. The short single cycle cleaning process can generally be performed even during the quickest of wafer inspections. The peak etching chamber pressure using the method and apparatus is kept below safety limits so that the existing process control interlocks do not need to be overridden.

[0008] The apparatus of the invention avoids the drawbacks of the HGS system by replacing the blower with an electrically heated, pressure regulated supply of nitrogen. The equipment for pressure regulation and heating takes up considerably less space than the blower and allows both pressure and temperature to be controlled for optimum performance. The method of the invention also uses existing equipment to initiate the cleaning cycle automatically, while the HGS system requires manual operation. However, other than using the existing signals, the method and apparatus do not interact (or interfere) with the regular etching process control, especially the interlocks.

[0009] The above objects, as well as additional objects, features, and advantages of the invention will become apparent in the following detailed description and in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic block diagram of an apparatus of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0011]FIG. 1 shows the preferred embodiment of the cleaning method 11 of the invention for use in cleaning a plasma etch chamber 13. The apparatus 11 includes a pressure sensor 15 to detect when the etching equipment is in an idle state, a cleaning gas line 17, a pressure regulator 19 and pressure gauge 21 installed in the cleaning gas line 17, an electrical heater 23 for heating the gas used to clean the etching chamber, a control valve 25 in the cleaning gas line 17 between the heater 23 and the etching chamber 13, and a process controller 27 to direct the cleaning operation. When a SECS/GEM port is available on the existing equipment, a digital signal taken from the SECS/GEM port is preferably used as the means for detecting when the etching equipment is in an idle state. The pressure sensor 15 can be used in the absence of a SECS/GEM port to inferentially detect the beginning of the etching chamber idle state. When the idle state begins, an existing nitrogen purge gas valve 29 is opened which causes a sudden rise of pressure in an existing purge gas supply line 31 [this purge gas supply line 31 is blocked off from the etching chamber 13 during installation of the apparatus of the invention]. Cleaning during the idle state is desirable because the normal process interlocks are disabled during the idle state, so there is no danger of having the safety protocols activated accidently during the cleaning cycle (e.g. by pressuring the etching chamber above a high pressure limit setting during the purge flow step to be discussed later).

[0012] The heater 23 is a special clean service unit made by Watlow Electric Manufacturing Company, located in St. Louis, Mo., designated Model CHETED1000KEKA. It has a two kilowatt power capacity and runs on standard 120 VAC electrical power. A pair of one kilowatt heaters connected in series has also been used, but a single larger capacity heater is preferred. A temperature sensor 33 such as a thermocouple or thermistor is located at or immediately after the outlet of the heater 23 and provides a signal to a temperature controller, which can be an independent apparatus, but preferably is part of the programming in the process controller 27. The temperature controller controls the operation of a power controller 35 that provides the 120 VAC power to the heater 23.

[0013] As previously discussed, a pressure regulator 19 is preferably provided to maintain tight control of the cleaning gas supplied to the etching chamber 13 through the cleaning gas line 17.

[0014] The method of the invention comprises a small number of steps that occur generally in sequence discussed below, although some of the steps can occur concurrently. In the first step, the apparatus 11 is in a standby state, while the process controller 27 awaits a signal from the pressure sensor 15 indicating that the etching chamber 13 has entered an idle state. During this standby state, there is no cleaning gas flow to the etching chamber 13. When the process controller 27 receives the desired signal from the pressure sensor 15, the controller 27 directs the on/off control valve 25 to open, causing cleaning gas to flow through the cleaning gas line 17 into the etching chamber 13 for a predetermined period, usually thirty seconds, during which time the pressure increases inside the chamber 13. After the predetermined period elapses the controller directs the on/off flow control valve 25 to close for a predetermined time while the etching chamber is evacuated by an existing evacuation pump 39 connected to the etching chamber 13. The flow and evacuation steps can be repeated as many times as desired, although when the etching chamber is idle for long periods, it is preferred that every twenty minute period of flow/evacuation steps be separated from the next twenty minute period of flow/evacuation steps by holding the evacuation step for about thirty minutes. The process controller 27 also is continuously performing the steps of monitoring the signal from the temperature sensor 33 and sending a control signal to the heater power controller 35 to adjust the rate of 120 VAC electrical power sent by the power controller 35 to the electrical heater 23. As an option, the process controller 27 can detect a signal 41 from an existing SECS/GEM port (not shown) indicating that the idle condition in the etching chamber has ended, whereupon the controller 27 resets the apparatus to its original standby state.

[0015] The invention has several advantages over the prior art. The apparatus and method share the advantages of the HGS system, such as longer MTBC (mean time between [wet] cleans), improved yield over time, etc. In addition, the apparatus can be constructed at a fraction of the cost of the HGS system, and takes much less time and cost to install. It has no complicated moving parts such as the roots blower in the HGS system, and can be started and stopped automatically and perform a complete cleaning cycle during wafer inspection when the etching chamber is idle. The apparatus can be operated on 120 VAC electrical power, unlike the HGS system which requires

[0016] The invention has been shown in only one embodiment. It should be apparent to those skilled in the art that the invention is not limited to the embodiment, but is capable of being varied and modified without departing from the scope of the invention as set out in the attached claims. 

1. An apparatus for cleaning a plasma etching chamber with a cleaning gas, comprising A) means for electrically heating the cleaning gas stream; B) means for detecting when the etching chamber enters an idle condition; C) means for controlling flow of cleaning gas to the etching chamber, and D) means for measuring and controlling the temperature of the cleaning gas that flows to the etching chamber.
 2. An apparatus as recited in claim 1, wherein the etching chamber has a purge gas line, further comprising a pressure sensor for detecting when the etching chamber enters an idle condition by sensing a rise in pressure in the purge gas line.
 3. An apparatus as recited in claim 1, further comprising means for regulating the pressure of the cleaning gas that flows to the etching chamber.
 4. An apparatus for cleaning a plasma etching chamber comprising A) an electrical heater B) means for detecting when the etching chamber enters an idle condition; C) a control valve connected in a cleaning gas line between the electrical heater and the etching chamber, and D) a temperature sensor located at or near the outlet of the electrical heater, and E) means for controlling the temperature of the cleaning gas that flows to the etching chamber.
 5. An apparatus as recited in claim 4, further comprising a pressure regulator installed in the cleaning gas line.
 6. A method for cleaning a plasma etching chamber used in manufacturing semiconductor devices with a cleaning gas, comprising the steps of: A) detecting when the etching chamber enters an idle condition; B) heating the cleaning gas; C) measuring and controlling the temperature of the heated cleaning gas; C) causing heated cleaning gas to flow into the etching chamber for a predetermined interval; and D) following step (C), stopping the flow of heated cleaning gas into the etching chamber and evacuating the etching chamber for a predetermined interval.
 7. A method for cleaning a plasma etching chamber as recited in claim 6, further comprising the step of regulating the pressure of the cleaning gas that flows into the etching chamber.
 8. A method for cleaning a plasma etching chamber as recited in claim 6, further comprising the step of stopping flow of cleaning gas to the etching chamber when the etching chamber initiates an etching cycle. 